THE QUEST OF PHYLETIC LINES 107 



them together into more extensive evolutionary series. In each 

 case the possibility of the series being progressive or retrogressive 

 or, it may be, even divergent from a central point, must be kept 

 in mind, and the line of criticism from physiological probability 

 be constantly applied. An error which is very frequently made 

 is to neglect the comparative study of species within a genus. 

 But it is only in this way that the short series above referred to 

 can be constructed, and such short series be ranged in any prob- 

 able relation one to another. A general, or generic conception of 

 the organisms compared will not suffice — a specific knowledge is 

 necessary for trustworthy comparisons. Or in other words, the 

 phyletic comparison should start upon the basis of specific knowl- 

 edge. It is this deficiency which underlies much of the modern 

 speculation as to the phyletic origin of Angiosperms. We shall 

 anticipate that those who have made the Angiosperms their 

 special study, while being adequately informed of the fossil data, 

 will be more likely to give a trustworthy solution of this phyletic 

 riddle than the students of fossils whose knowledge of the living 

 flora of Angiosperms is rarely wide and at the same time specific. 

 It has been already pointed out how deficient is the early fossil 

 record, and how isolated are those plant types which we have rea- 

 son to regard as the most primitive. Such survivals are, as a 

 rule, poor in species, or may even be represented by only one. 

 This makes comparison still more difficult. But turning from 

 these, in reference to which comparative morphology is sometimes 

 little better than guess-work, there are certain large groups in 

 which the comparison is susceptible of more scientific treatment . 

 The factors which lead to this more happy result are: first, the 

 representation of the phylum by numerous living families and 

 genera, with a good spread of species; secondly, that the charac- 

 ters on which the classification is based shall be as various as possi- 

 ble; thirdly, the fossil origin should not be too remote, and the 

 record should extend through as long a sequence of geological 

 horizons as possible. Given these conditions, an adequate study 

 of the representatives, conducted specifically, and based primarily 

 on the living forms, with the data from the fossils serving as a 

 check, should lead to sound views as to descent within the phylum, 



